The present invention relates to a thin film electroluminescence device, and more particularly to an emitting electroluminescence device capable of emitting white light. In order to obtain white light emission by use of a conventional electrolumincescence device, the following three methods have been proposed:
(1) Adding a single activator for white light emission to a single host material for attainment of plural electron transitions.
(2) Adding plural activators to a single host material so as to perform additive mixing of different emitted lights to produce white light.
(3) Overlaying a plurality of electroluminescence (hereinafter referred to as EL) emitting layers, each layer comprising a single activator and a single host material, so as to add each EL emission light to produce white light.
The above methods, however, have the following shortcomings. With respect to the first method, praseodymium(Pr) is the only activator available at present praseodymium(Pr) is the only activator available at present for use in the first method and the brightness obtained by this method is insufficient for use in practice.
The second method may be possible, but there has not been discovered yet a combination of a plurality of activators and a single host material that can produce white light by addition of each EL light produced from each activator. Even if such a combination is discovered, it is well predicted that the total brightness obtained will be extremely small due to the interaction of the activators employed, or it will be extremely difficult to harmonize the threshold of the electric field for each EL emission to produce white light emission.
In view of the problems of the above-mentioned first and second methods, the third method appears most promising. In fact, many proposals have been made concerning the overlaying of EL emitting layers according to the third method. However, the conventional third method has the shortcomings that (i) sufficient brightness is not available in the so-called blue EL emission with the emission wavelengths ranging from 400 nm to 500 nm, (ii) the voltage-brightness characteristics are so different between each EL emitting layer that usable white light is not obtained by the addition of the light emitted from each EL shortcomings, a practically usable EL device according to the third method has not been obtained yet.